I'm using XenTag as the tag add-0n and also there is many forums even another forum softwares which using this kinda add-ons. But I'm not sure how this tags effective? How helpful to seo? Should I keep using this add-on?
Tags provide a huge benefit for SEO, because, among other things, they are very helpful with PageRank sculpting and getting the right keywords/anchor text on the page. Without them, some of your best old threads can have almost not PageRank as soon as the fall of the first page of results. Likewise, Google may decide, based on textual and link analysis that your site is about the usernames of your most prolific posters rather than it's actual subject matter (it happened to me; check Google Webmaster Tools).
The problem with XenTag is that it isn't getting updated frequently enough. It's "good enough," but it is missing the 3 following key features:
While the product is "good enough" without the above features, and many people use it successfully (including myself), the absence of these features is a matter of significant concern, given how vital the project is to the communities it is used on. The developer is not "responsive enough" for this to not be in the core. In the developer's defense, they did add hashtagging recently, but, in my mind, the mark of a mature product is how well it handles the basics.
The above links also explain why I believe in the importance of a curated list of tags. The features I link to above are vital for a curated list of tags. If others are willing to join in, I'd be willing to pay to help develop some of the features, but really they aren't something one should have to pay more for in a product this old, this widely used, and this important to a forum.